As is known, in-home/in-building networks and point-to-point wireless communications occur between two or more wireless communication devices such as laptop computers, personal computers, personal digital assistants, Internet connections, hand held radios, et cetera. Such wireless communication devices include a transmitter section and a receiver section. In general, the transmitting section includes a modulator, an up-conversion intermediate frequency (IF) stage, and a power amplifier to drive an antenna. The receiving section generally includes a low noise amplifier operably coupled to an antenna, a down-conversion IF stage, and a demodulator.
To transmit data from one wireless communication device to another, the modulator of the transmitting section of the initiating device modulates the data to produce modulated data. The up-conversion IF stage mixes the modulated data with a local oscillation to produce an RF signal that is amplified by the power amplifier and transmitted via the antenna. A local oscillator generates the local oscillation from a crystal oscillator circuit within the initiating device.
The receiving section of the targeted device receives, via its antenna, the RF signal, which is amplified by the low noise amplifier. The RF signal is then mixed with a local oscillation via the down-conversion IF stage to produce an IF signal or base-band signal. The demodulator demodulates the IF signal or base-band signal to recapture the data. A local oscillator in the receiving section of the receiving device generates the local oscillation that is used for the down conversion of the RF signal. The particular modulation scheme and subsequent demodulation scheme used by the initiating device and targeted device is dependent on the particular wireless communication protocol adhered to by such devices. For example, the wireless communication protocol may be Bluetooth, IEEE 802.11a, IEEE 802.11b, code division multiple access (CDMA), analog mobile phone service (AMPS), digital AMPS, global system for mobile (GSM), wireless application protocol (WAP) and/or any other wireless communication standard.
Typically, the local oscillation used by the receiving section and transmitting section in each wireless communication device is derived from a crystal oscillator and a phase locked loop. Since crystal oscillators are generally inaccurate devices (e.g., have an error of +/−5%), the local oscillation produced by one wireless communication device may not be the same local oscillation as produced by another wireless communication device. When this occurs, a DC offset results in the receiving section of the targeted wireless communication device. Such a DC offset can cause errors in the recapturing of the data. The errors caused by the DC offset may be magnified when the wireless communication devices employ complex modulation schemes.
As is further known, in-home/in-building networking and point-to-point wireless communications are governed by a variety of standards including Bluetooth, IEEE802.11a, IEEE802.11b, et cetera. Each of these standards provides guidelines for encoding/decoding and/or modulating/demodulating data. In addition, the standards specify a frequency band for the wireless conveyance of data. For example, the IEEE 802.11a standard specifies a frequency band of 5.15 gigahertz to 5.35 gigahertz and 6.725 gigahertz to 5.825 gigahertz and further specifies a modulation scheme of orthogonal frequency division multiplexing (OFDM).
For a wireless communication device to be compliant with the IEEE 802.11a standard, it must generate, for a direct down-conversion, a local oscillation that has a range of 5.18 gigahertz to 5.32 gigahertz and also a range of 5.745 gigahertz to 5.805 gigahertz. In addition, the local oscillation should be accurate within a few kilohertz such that it can be centered in the 16.25 megahertz bandwidth of each channel, which consists of 312.5 kilohertz sub-channels. Further, the local oscillation should be able to change its frequency very quickly to accommodate the channels. Consequently, a phase locked loop within the local oscillator cannot be used to adjust the frequency of the local oscillation since it is too slow (e.g., a PLL has a bandwidth of up to 60 kilohertz).
Therefore, a need exists for a method and apparatus of generating a self-correcting local oscillation that is accurate to within a few kilohertz and may be changed quickly to provide a range of local oscillations.